工程目录：

清单文件增加访问网络权限，防止Cleartext HTTP traffic to xxx not permitted错误设置CleartextTraffic=“true”

<uses-permission android:name="android.permission.INTERNET"/>

        android:usesCleartextTraffic="true"

Activity

package com.example.ex_eight;

import androidx.appcompat.app.AppCompatActivity;

import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class MainActivity extends AppCompatActivity {
    private Button btn;
    private TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btn=findViewById(R.id.btn);
        tv=findViewById(R.id.tv);

        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {   //事件处理
                new HttpTask().execute();
            }
        });
    }

    class HttpTask extends AsyncTask<Void,Void,String> {
        @Override
        protected String doInBackground(Void... voids) {  //耗时操作代码在后台进行
            String httpUrl = "http://www.zlm.design"; //域名字符串
            String resultData = "";  //结果字符串
            URL url = null;  //URL对象
            try {
                url = new URL(httpUrl); //构造URL对象时需要使用异常处理
            } catch (MalformedURLException e) {
                Log.i("TAG", "URL对象创建失败！");
            }
            if(url != null) {  //如果URL不为空时
                try {
                    //有关网络操作时，需要使用异常处理
                    HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();   // 打开连接并转型
                    //服务器返回数据字符流，网页文档编码一般为UTF-8或GBK
                    InputStreamReader in = new InputStreamReader(urlConn.getInputStream(), "UTF-8");
                    //为输出创建BufferedReader
                    BufferedReader buffer = new BufferedReader(in);
                    String inputLine = null;
                    while (((inputLine = buffer.readLine()) != null)) {
                        resultData += inputLine + "\n";   //换行
                    }
                    in.close(); //关闭输入流
                    urlConn.disconnect(); // 关闭HTTP连接
                } catch (IOException e) {
                    resultData=e.getMessage();
                }
            } else {
                resultData = "url is null"; //当url为空时输出
            }
            return resultData;
        }
        @Override
        protected void onPostExecute(String resultData) {  //在后台数据提交后更新UI主线程
            if (resultData != null)
                tv.setText(resultData);  //原生用法，更新UI工作在onPostExecute()方法里
            else
                tv.setText("Sorry,the content is null");
        }
    }
}